1. Field of the Invention
The present invention relates to an inverted telephoto type wide angle lens system for a single lens reflex camera, and more particularly to an improvement in a focusing system thereof.
2. Description of Prior Art
In the field of interchangeable lens systems for single lens reflex cameras, a wide angle lens system is generally of an inverted telephoto type having a negative front lens group for securing a sufficient back focal distance thereby enabling the flip up motion of the reflex mirror. The focusing in such an inverted telephoto type wide angle lens system has been generally practiced by means of shifting the whole lens system. In close focusing, however, the image is seriously disturbed in the zonal to marginal area because of the asymmetric power distribution with respect to the aperture stop specific to this type of lens system. Especially, the over-correction of astigmatism and the under-correction of spherical aberration are apt to be caused upon close focusing.
To overcome the above mentioned disadvantage, one or more necessary air spaces in the inverted telephoto type lens system have been conventionally changed in association with the shifting of the whole lens system for compensating the deterioration of aberration correction in close focusing. This compensation system, however, has required both a lens barrel mechanism for shifting the whole lens system and a lens barrel mechanism for changing the one or more air spaces cooperative with the whole lens shifting mechanism, to result in a total lens barrel mechanism of a highly complex and bulky type in comparison with an ordinary lens barrel mechanism having no compensation system.
On the other hand, various inverted telephoto type wide angle lens systems have been suggested in the prior art such as those disclosed in U.S. Pat. Nos. 4,029,397 and 4,099,849 and German Patent Application Laid Open No. DOS 2,946,182.
The consumer requirements of the camera industry have placed increasing demands on lens designers to provide ever increasing optical performances while further compacting and making lightweight lens systems. Accordingly, even though the prior art is crowded in attempts to achieve improved optical performance, there is still a demand to provide an improved inverted telephoto type wide angle lens system.